1. Field of the Invention
The invention relates to cylinder block provided with a coolant passage inside of an inter-bore partition wall that is positioned between adjacent cylinder bores, and to a manufacturing method of the cylinder block.
2. Description of Related Art
An inter-bore partition wall that is positioned sandwiched between cylinder bores that form combustion chambers is easily affected by combustion heat, and thus tends to reach a high temperature as an engine operates. Therefore, a cylinder block in which a coolant passage for guiding some coolant in a water jacket is provided inside the inter-bore partition wall is known.
In order to reduce the weight and size of an engine, it is preferable that each portion of the cylinder block be thin, and that the inter-bore partition walls also be as thin as possible. The center portion of the inter-bore partition wall where adjacent cylinder bores are closest is a portion where cooling is particularly important. However, the inter-bore partition wall is thin, so if a coolant passage that enables coolant to pass through this center portion is formed, strength is no longer able to be ensured. Therefore, if the inter-bore partition wall is made thin in order to reduce the weight and size of the cylinder block, a coolant passage that passes through the center portion of the inter-bore partition wall may not be able to be provided.
Therefore, a cylinder block described in Japanese Patent Application Publication No. 9-151784 (JP-A-9-151784) is provided with a coolant passage that is curved by joining an upper passage that extends away from the center portion toward a cylinder head side with a lower passage that extends away from the center portion toward a crankcase side, with the upper end of the lower passage being communicated with the lower end of the upper passage. Coolant in the water jacket is guided near the center portion of the inter-bore partition wall by this curved coolant passage.
Employing this structure makes it possible to guide coolant near the center portion without forming a coolant passage that passes through the center portion of the inter-bore partition wall, so the center portion of the partition wall is able to be cooled while still ensuring the strength.
With the cylinder block described in JP-A-9-151784, the upper passage is formed by drilling a hole at an angle from a portion on the cylinder head side inside the water jacket toward the center portion side of the inter-bore partition wall. Meanwhile, the lower passage that communicates the water jacket with the upper passage is formed by drilling a hole through the water jacket from inside the crankcase toward the cylinder head side. After the lower passage is formed in this, way, the unwanted through-hole that remains on the crankcase side is blocked off (see paragraph [0014] and FIG. 6 in JP-A-9-151784).
With the cylinder block described in JP-A-9-151784, an unwanted through-hole is formed in the process of forming the coolant passage, so a process to block off this unwanted through-hole is required.
Also, the coolant passage described in JP-A-9-151784 communicates the upper and the lower portions of the water jacket through the inside of the partition wall, and circulates coolant from down to up using natural convection that increases as the temperature of the coolant in the coolant passage rises (see paragraph [0017] in JP-A-9-151784). Therefore, with the cylinder block described in JP-A-9-151784, even if a flow is generated in the coolant inside of the Water jacket, a flow is not easily generated in the coolant inside the coolant passage, and thus coolant inside the coolant passage does not readily circulate.